X-ray inspection systems can be used for screening objects, such as cargo pallets, crates, and containers, to detect the presence of explosives and other prohibited items. Scanning containers, for example, can allow the contents to be examined in a non-invasive manner, i.e., without opening or physically inspecting the container.
In an x-ray scanning system, x-rays can be directed at the object in question. Depending on the density and composition of the object, some of the energy carried by the x-rays can be deposited in or reflected by the object, the energy thereby failing to be transmitted through the object being referred to as absorbed energy, or the absorbed dose. Transmitted x-rays can then be measured by a detector located proximate to the object and opposite the x-ray source. Among other factors, the attenuation of the transmitted radiation can be a function of the density of the material being scanned. Data received by the detector can be used to produce a radiographic image that that can be correlated with the shape, size, and density of the scanned object.
In addition, data received at the detector can be used to determine an absorption coefficient and effective atomic number of materials associated with the scanned object. Materials of interest, including, for example, explosive materials can be identified by comparing the absorption coefficients of the acquired data with known absorption coefficients of known materials of interest.
Both the size and density of the scanned object can affect the resultant x-ray data. The contents of containers scanned in a shipping setting, for example, can vary widely in both density and composition. For example, x-ray scanning systems that use the same total emitted radiation, i.e., high-energy or low-energy, regardless of the object being scanned, can generate data or images of poor quality that cannot be used to accurately identify items of interest.
In general, “low-energy” x-ray scanning systems can operate within the energy range of 75 kV to 200 kV. Due to the low power of these sources, inspection can be limited to small, low-density containers, such as those containing, for example, mail, flowers, or clothing. “High-energy” scanning systems, those that can generally operate in the range of 300 kV to 10 MV, can be used to inspect larger, higher-density items and containers, such as those containing, for example, machine components, construction materials, or electronics. Therefore, a low-energy scanner can exhibit insufficient x-ray penetration to effectively screen high-density objects. Similarly, a high-energy scanner, can fail to produce an adequate image of a low-density object.